Electrochromic materials have the property of changing their colour depending on the charging state. Electrochromic devices have been developed for a number of different applications, ranging from e.g. eyewear to displays. One typical application is in so-called smart windows. By changing a voltage applied across a film provided at a window glass, the window can be made dark and thereby it significantly reduces the amount of transmitted light. This is of particular interest where the amount of solar radiation is large and one wants to reduce the amount of light and/or heat being transmitted into the room inside the window. A fast transition between dark and light appearance is often required, enabling control of the colour state of the window depending e.g. on whether a person is present in the room or not. Also as complete a transition as possible, i.e. from very transparent to almost black, is requested.
Even if the control of the transmission of the visible light is satisfactory with electrochromic devices of today, heat transfer through the windows may still cause problems. In a majority of cases where windows provided with electrochromic devices are used, the need for achieving heat from the light through the window is generally low. On the contrary, in most cases the heat transfer through prior art smart windows is too large. When the window is transparent, it is not always requested or even wanted to have the infra red radiation transmitted into the room. Thus, there are still problems with too high heat transfer through prior art electrochromic devices in different applications.